Well cementing is a process used in penetrating subterranean zones (also known as subterranean formations) that produce gas. In well cementing, a well bore is drilled while a drilling fluid is circulated through the well bore. The circulation of the drilling fluid is then terminated, and a string of pipe, e.g., casing, is run in the well bore. The drilling fluid in the well bore is conditioned by circulating it downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore. Next, primary cementing is typically performed whereby a slurry of cement in water is placed in the annulus and permitted to set into a hard mass to thereby attach the string of pipe to the walls of the well bore and seal the annulus.
The movement of gas from the subterranean zone into and through the cement slurry during and after primary cementing is known as gas migration. Gas migration in the annulus can cause severe problems, for example, high volume loss of gas from a high pressure zone to a low pressure zone and the failure of the cement to seal the annulus, which can undesirably lead to an uncontrollable blow-out.
Gas migration is caused by the behavior of the cement slurry during a transition phase in which the cement slurry changes from a true hydraulic fluid to a highly viscous mass showing some solid characteristics. When first placed in the annulus, the cement slurry acts as a true liquid and thus transmits hydrostatic pressure. During the transition phase, certain events occur that cause the cement slurry to lose its ability to transmit hydrostatic pressure. One of those events is the loss of fluid from the slurry to the subterranean zone. Another event is the development of static gel strength (i.e., stiffness) in the slurry. When the pressure exerted on the formation by the cement slurry falls below the pressure of the gas in the formation, the gas initially migrates into and through the cement slurry. The gas migration causes flow channels to form in the cement slurry, and those flow channels permit further migration of the gas after the cement slurry sets (i.e., hardens into a solid mass).
Various techniques have been developed for eliminating undesirable gas migration. For example, U.S. Pat. No. 5,327,969 discloses a method in which the initial surface pressure in the pipe is determined after the cement slurry is placed in the annulus, followed by displacing additional cement slurry into the annulus as is necessary to maintain the pipe surface pressure substantially equal to the initial surface pressure in the pipe; U.S. Pat. No. 5,339,903 discloses a method in which a compound consisting of a tannin backbone having polymers grafted thereto is added to a slurry of cement in water to reduce fluid loss from and to modify the gel strength of the slurry; and U.S. Pat. No. 5,503,227 discloses a method in which one or more lateral openings are formed through the casing and the cement sheath into the subterranean formation, one or more horizontal fractures are created in the formation extending from the lateral openings, and a fluid that sets into a substantially gas impermeable solid is deposited in the openings and fractures, thereby plugging passages in the cement sheath and terminating gas migration.
Unfortunately, conventional attempts to prevent gas migration during primary well cementing have been unreliable, difficult to carry out, and/or very expensive. As such, there continues to be a need for improved methods for eliminating gas migration during well cementing. The present invention utilizes a relatively inexpensive and simple method to inhibit gas migration.